2D Water Marbling (Ebru): Marangoni Paint Spreading

Tool

Ink colour

Paint & water

Surface tension gap (Δγ) 65

Drop volume medium

Comb width 14 mm

Readouts

Drops placed

Active fronts

Front speed dr/dt = k / r

— mm/s

How it works

Same physics as the 3D water-marbling engine, in a top-down 2D paint field:

Marangoni spreading — ebru paints are formulated with a surface tension lower than water's. That tension gap γ_water − γ_paint pulls the drop outward along the surface; drag from the thinning film grows with the wetted area, so the front slows as it grows: dr/dt = k / r, giving r(t) = √(2kt).
Film thinning — pigment volume is roughly conserved as the ring expands, so ink density falls off as ≈ 1/r — later rings read visibly thinner and more transparent than the fresh drop centre.
Concentric / interleaved rings — each new drop's expanding front is a real outward flow that physically displaces (advects) paint already on the surface, pushing older colours into rings around it — the same vector-field advection used for the comb, just radial instead of directional.
Comb / stylus — dragging the tool advects the whole paint-colour field along the tool's velocity within its reach, folding straight rings into the swirled, feathered ebru patterns without adding or removing any pigment.